This invention relates to the measurement of concentrations of non-volatile residue in liquids, and more particularly to systems and processes for determining non-volatile residue concentrations in solvents.
It is known that the fabrication of very large scale integrated (VLSI) circuits requires an abundance of ultrapure water. More particularly, a complete fabrication process may involve over fifty stages of processing the surface of the semiconductor wafer. A washing with the ultrapure water follows each stage of processing, for removal of chemicals used in that stage. Accordingly, thousands of liters of ultrapure water may be used in processing a single wafer. Any non-volatile residue present in the ultrapure water can remain on the surface of the wafer after the water has evaporated, possibly causing defects in the resulting semiconductor device. This gives rise to a need to monitor the ultrapure water for the presence of non-volatile residue, to insure that the concentration of such residue remains at or below an acceptable level.
Similarly, there is a need to determine the non-volatile residue concentration in various solvents used in etching, deposition, cleaning and other stages of fabrication. Herein, "solvents" is used generically to include organic solvents such as isopropyl alcohol and acetone, and inorganic solvents such as hydrochloric acid, hydrofluoric acid, ammonium hydroxide, hydrogen peroxide and water. These solvents must be tested to determine their purity. Further, as to solvents used in cleaning, it is advantageous to measure residual contamination extracted from components that have been cleaned in the solvent, as an indication of the degree to which such components have been cleaned, and as an indication of whether the solvent remains suitable for cleaning further components.
Systems have been developed and employed successfully in continuously monitoring the quality of ultrapure water. For example, U.S. Pat. No. 5,098,657 (Blackford et al) discloses an apparatus for measuring non-volatile residue concentrations in ultrapure water. Fixed and adjustable flow restrictive elements are arranged to provide a constant, pressure controlled flow of the water to an atomizer. At the atomizer the water is formed into droplets which are later dried to provide non-volatile residue particles. An electrostatic aerosol detector determines the particle concentration, which provides an indication of the purity of the water.
Solvents, however, do not lend themselves to this type of continuous flow system, in which the fluid flows at a rate of at least fifty milliliters per minute. Due in part to their volatility, and in the case of acids their corrosiveness, solvents give rise to safety concerns in their handling, release vapor emissions, and create waste disposal problems. Accordingly, solvents preferably are used and tested in the lowest workable amounts and concentrations.
The conventional method for testing solvents for non-volatile residue is to evaporate a measured quantity of the solvent in a previously weighed container. The original volume of liquid and the weight of material remaining after evaporation, are used to compute residue concentration. Given the need to determine residue concentrations in the single part per billion range, a relatively high volume of the solvent (e.g. one liter) is required for an accurate measure of concentration. The testing procedure is time consuming in view of the need to completely evaporate the solvent. This approach is costly, yet can not provide real time residue concentration data. Such testing gives rise to difficulties in solvent handling, potentially harmful vapor emissions, and waste disposal problems.
While the above discussed needs and difficulties in ascertaining solvent purity are perhaps particularly apparent in connection with fabrication of semiconductor devices, they arise in other industries, e.g. manufacture of disk drives and recording media, precision optics, inertial guidance and aerospace applications.
Therefore it is an object of the present invention to provide a system and process for accurately determining levels of impurities in solvents by testing extremely small quantities of the solvents.
Another object of the invention is to provide a simple and rapid means for obtaining real time information on the concentration of non-volatile residues in volatile solvents.
A further object is to provide a low cost approach to monitoring contamination levels of cleaning solvents used in semiconductor wafer processing and other manufacturing techniques that require exceptionally clean parts.
Yet another object is to provide a process for testing contamination levels in solvents employed during various stages of semiconductor wafer processing (and other processes), with equipment already utilized in monitoring contamination levels in ultrapure water.